1. Field of the Invention
The present invention relates to a mobile communications system, and more particularly, to a method for performing reverse link power control by determining the state of a reverse link data rate control bit according to variable thresholds that are set according to the size of a data packet to be transmitted.
2. Description of the Related Art
Mobile communication systems generally provide a reverse link packet data channel via which packet data may be transmitted in a reverse link. The R-PDCH (reverse packet data channel) of the CDMA-2000 system is an example of such a reverse link packet data channel. Transmission at 1.5 Mbps is enabled via R-PDCH. The R-PDCH, the Forward Acknowledgment Channel (F-ACKCH), the Reverse Packet Data Control Channel (R-PDCCH), the Forward Indicator Channel (F-ICCH), the Reverse Request Channel (R-REQCH), and the Forward Grant Channel (F-GCH) are all supported in the CDMA system.
The reverse link data rate in the CDMA system is associated with power control. Power control is a method for enabling uniform reception signal power in order to meet specific performance requirements while decreasing interference with other users. Reverse link power control in the CDMA system may be accomplished by maintaining uniform reception power for a reverse pilot channel (R-PICH).
Specifically, a base station sets a threshold and measures the reception power level of the R-PICH. If the measured level is higher than the threshold, the base station delivers a power control bit (PCB) indicating ‘DECREASE’ to a mobile station. If the measured level is lower than the threshold, the base station delivers a power control bit indicating ‘INCREASE’ to the mobile station. The mobile station, having received and determined the state of the power control bit, adjusts the transmission power of its R-PICH.
Power control of data or control channels is performed by enabling other reverse channels to have a uniform ratio with respect to the transmission power level of the R-PICH. In order to maintain a uniform ratio, the transmission power of each of the channels transmitted from the mobile station except the R-PICH is adjusted by utilizing a traffic-to-pilot power ratio (TPR) that is defined as the ratio of transmission power of the corresponding channel to that of the R-PICH. The TPR is a predefined value set to meet the requested performance of the corresponding channel and is determined, for example, based on a data rate, a coding scheme, and a modulation scheme.
The mobile station does not arbitrarily determine the TPR of the R-PDCH. If a specific mobile station arbitrarily uses a high TPR, serious interference with other mobile stations cause the system to become unstable. To prevent such interference, a reverse TPR of the mobile station is adjusted by considering a state of a reverse channel established between the base station and each mobile station.
In conventional methods using rate control as a means of utilizing reverse radio resources, a base station assigns a forward rate control channel (F-RCCH) and transmits a reverse link data rate control bit (RCB) to each mobile station. The RCB may indicate one of two or three possible states, for example an ‘INCREASE’ state in which a mobile station raises its TPR, a ‘DECREASE’ state in which a mobile station lowers its TPR, and a ‘HOLD’ state in which a mobile station maintains its current. TPR.
To determine the TPR of the R-PDCH, a mobile station utilizes a RCB received from a base station and a parameter, such as authorized_TPR, indicating a new TPR to set. FIG. 1 illustrates a variation of the TPR of the R-PDCH according to a conventional data rate control method. A base station transmits an RCB in a specific (ith) frame based on a TPR of a previous ((i-β)th) frame and a mobile station sets a TPR in a future ((i+α)th) frame based on the received RCB. The parameters α and β are integers equal to or greater than 1 and represent a time delay in the course of transmission and reception between the base and mobile stations. According to the current cdma2000 standard (Revision D), both α and β are set to 2.
When the mobile station receives an RCB via F-RCCH, the authorized_TPR is determined as follows:                1) authorized_TPR=authorized_TPR_past+UP_step[I] (if RCB indicates ‘INCREASE’)        2) authorized_TPR=authorized_TPR_past+DOWN_step[I] (if RCB indicates ‘DOWN’)        3) authorized_TPR=authorized_TPR_past (if RCB indicates ‘HOLD’)        
The authorized_TPR parameter indicates a TPR corresponding to a specific data rate index (I). UP_step[I] indicates a TPR increased by one step from ‘I’. DOWN_step[I] indicates a TPR decreased by one step from ‘I’. The authorized_TPR_past parameter is the previous TPR (authorized_TPR).
A mobile station determines a data rate at which to transmit according to the newly determined TPR (authorized_TPR). The mobile station stores the new TPR value as authorized_TPR_past for use in determining the next TPR (authorized_TPR).
Since the RCB may have three states, the RCB state may be indicated in the following manner when transmitting the RCB from the base station:                1) RCB signal phase set to ‘−’ to indicate ‘INCREASE’ state        2) RCB signal phase set to ‘+’ to indicate ‘DECREASE’ state        3) RCB signal is not transmitted to indicate ‘HOLD’ state.        
Utilizing the conventional data rate control method, stability of a system may depend upon the reliability of the RCB transmitted via F-RCCH to a mobile station. If a mobile station incorrectly determines the state indicated by the RCB, for example by determining that the received RCB indicates ‘HOLD’ state when the base station transmits an RCB indicating ‘DECREASE’ state, the mobile station may maintain the previously authorized TPR when the TPR should be decreased and unexpected resource utilization may cause the reverse link system to become unstable. Therefore, the thresholds for determining the state (‘INCREASE’, ‘HOLD’, or ‘DECREASE’) of the RCB transmitted from the base station should be set in order to minimize the potential for instability of the system.
FIG. 2 illustrates a conventional method of determining the state of a RCB signal transmitted to a mobile station from a base station. A mobile station determines one of ‘INCREASE’ state, ‘DECREASE’ state, and ‘HOLD’ state utilizing a first threshold value (TH_τ) for an RCB reception signal size and a second threshold value (TH_θ) for an RCB reception signal phase.
If the RCB reception signal size is smaller than the first threshold (TH_τ), the mobile station determines that a received RCB signal indicates ‘HOLD’ state. If the reception signal size is greater than the first threshold (TH_τ) and the reception signal phase lies between the inverse of the second threshold (−TH_θ) and 180° added to the second threshold (180°+TH_θ), the mobile station determines that the received RCB signal indicates ‘INCREASE’ state. If the reception signal size is greater than the first threshold (TH_τ) and the reception signal phase lies between 180° added to the second threshold (180°+TH_θ) and the second threshold subtracted from 360° (360°−TH_θ), the mobile station determines that the received RCB signal indicates ‘DECREASE’ state.
When setting the first (TH_τ) and second (TH_θ) thresholds for determining the state of the RCB, the mobile station considers a “missing probability” and “false alarm probability”. The “missing probability” is the probability of incorrectly determining an RCB transmitted by the base station to indicate ‘INCREASE’ state as indicating ‘HOLD’ state or ‘DECREASE’ state. The “false alarm probability” is the probability of determining an RCB transmitted by the base station to indicate ‘DECREASE’ state as indicating ‘HOLD’ or ‘INCREASE’ state.
Generally, for system stabilization, it is preferable that the first (TH_τ) and second (TH_θ) thresholds are set to have a small “false alarm probability” since the mobile station incorrectly increasing or maintaining the data rate due to an incorrect determination when the data rate should be decreased (“false alarm”) has a greater affect on system stabilization than the mobile station incorrectly decreasing the data rate due to an incorrect determination when the data rate should be maintained or increased (“missing”). However, if the “false alarm probability” is set too small, the “missing probability” is increased, thereby making it more likely that the mobile station will incorrectly determine the state of an RCB indicating ‘INCREASE’ state for which the TPR should be increased and this possibility should be considered when setting the first and second thresholds.
Table 1 illustrates a reference value (normal_TPR) according to the size (EP_SIZE) of encoder packet that is to be transmitted in the physical channel according to the current CDMA standard. As illustrated in Table 1, as the size (EP_SIZE) of the encoder packet increases, the higher the transmission power is increased and the greater the difference between sections becomes. Generally, as the size (EP_SIZE) of the encoder packet increases, the greater the effect on the stability of the system that an incorrect determination of the state of an RCB transmitted from the base station to a mobile station has.
However, in conventional methods, the first (TH_τ) and second (TH_θ) thresholds are set constant regardless of the size (EP_SIZE) of the encoder packet to be transmitted. Therefore, the greater sensitivity of the system to an incorrect determination of the RCB signal state by a mobile terminal is not taken into consideration.
TABLE 1(Pilot Power = 0 dB)normal_TPRPowerDifferenceRow IEP_SIZE[dB][Watt][Watt]11920.751.1824083.752.371.1937926.754.732.36415609.6259.174.445309611.87515.46.236463213.62523.047.647616814.87530.737.698924016.62545.9715.249123121863.0117.04101538419.12581.7518.74
Therefore, there is a need for a method of determining the state of an RCB signal that minimizes the effect that the size of a data packet to be transmitted has on the sensitivity of system stability to incorrect determinations of the RCB state by a mobile station. The present invention addresses these and other needs.